The fusion function of enveloped viruses has been intimately tied to the initial steps in infection of susceptible host cells. In all systems studied so far the outer proteins, usually glycosylated, have been implicated in fusion. Because fusion occurs at an early point in the infectious process, blockage with either antibodies or antiviral agents has potential application to the prevention and treatment of viral diseases. The fusion function of bunyaviruses was first demonstrated in our laboratory and this proposal will explore biological and molecular aspects of fusion using California serogroup bunyaviruses. Existing data indicate that, of the two envelope glycoproteins G1 and G2, the G1 protein plays a role in fusion. However, the protein which carries the putative hydrophobic fusion sequence is yet to be determined. We propose the following studies with La Crosse and other California serogroup viruses. 1) Liposomes containing one or both viral glycoproteins will be constructed and tested for fusion capability to identify the protein responsible for fusion; 2) The M RNA gene segment coding for the viral glycoproteins will be cloned and sequenced in order to identify the region(s) of the glycoprotein that mediate fusion; 3) Fusion mutants will be generated and compared to fusion-altered antigenic mutants and to parental LAC virus to identify genome sequences associated with the fusion function; and 4) The role of fusion in infection of mice will be explored by characterizing the pathogenesis of fusion mutants that have altered neuroinvasiveness and by in vivo use of lysosomotropic agents that are though to interfere with fusion.